Lack of data on the space position of the axes of the now-operating cased wells and on an accurate position of their bottomholes in the pattern of the oil or gas fields under development prevents one from developing such fields at a required technical level.
The bores of the wells of the aforesaid operating well stock precludes application of the use of geomagnetic fields for determining an azimuthal deviation of the well axes. However, too sophisticated construction, high cost, and inadequate accuracy of the gyroscopic inclinometer systems which make use of the principal axis of a gyroscopic system for determining an azimuthal deviation of the well axis, impede one to solve said problem to a sufficient extent.
A prior-art method and device for determining a space position of the longitudinal axis of a cased well U.S. application Ser. No. 4,192,077, filed Mar. 11, 1980. The method provides for obtaining output data of azimuthal measurement, using a free gyroscope and a rate-of-turn gyroscope during displacement of equipment in the well being surveyed.
It is evident that a combination of a free gyroscope and a rate-of-turn gyroscope contributes to the fact that advantageous features of each of said gyroscopes (i.e., an adequate accuracy of the rate-of-turn gyroscope and a higher surveying rate of the free gyroscope give as total result exceeding that of the two gyroscopes taken individually.) For instance, in the event of a power interruption, a tumbled free gyroscope can be reoriented by using the output date of the rate-of-turn gyroscope, obviating any need to bring the free gyroscope back into the well for realignment.
However, use of an azimuth of the principal axis of the gyroscopes results in an azimuthal error, since the space position of the principal axis of each gyroscope is affected adversely by dynamic loads the gyroscopes are exposed to during the round trip of equipment, rotation of the Earth, and some other factors that are hardly amenable or unamenable altogether to elimination. Furthermore, the gyroscopes in question feature a sophisticated construction arrangement which adds much to the cost of the process of directional surveying of wells.
Other prior-art methods and devices for directional surveying of cased well are known (cf. a textbook "Directional surveying of wells" by V.Kh.Isachenko, Moscow, Nedra PH, 1987, pp. 17-20, 78-83 (in Russian).
The aforementioned method is carried into effect with the aid of a down-the-hole instrument-inclinometer, and a ground-level until for receiving, processing, and displaying the output data obtained from said inclinometer.
The housing of the instrument accommodates a sensor of the zenith angle of the well being surveyed and a gyroscopic system for determining the azimuth of the well.
It is due to the aforesaid gyroscopic system retaining the space direction of its principal axis that enables one to measure the well azimuth without using the geomagnetic field, that is, in cased wells.
However, the space position of the gyroscopic system principal axis is affected adversely by dynamic loads the instrument is exposed to during its round trip to the well being surveyed, as well as by rotation of the instrument round its longitudinal axis, rotation of the Earth, and the like factors, which change the space position of said axis that serves as the datum point of the azimuthal measurement. This in turn involves a considerable error (of the order of plus-minus 10.sub.0) in azimuth determination. On the other hand, measures taken to eliminate said adverse factors necessitate inevitably increased overall dimensions of the gyroscopic system and hence those of the down-the-hole instrument as a whole, which is far from being always practicable under conditions of each specific well, or lead to a constructional sophistication of the gyroscopic system and hence to a much higher cost of the device. Moreover, use of the gyroscopic system involves practical implementation of the method in question more technologically complicated, which is due to a prolonged period of tuning the system and of the process proper of directional surveying of the well being surveyed.